First-trimester screening for aneuploidy: Research or standard of care?

First-trimester screening for Down syndrome has been proposed as a signific ant improvement with respect to second-trimester serum screening programs, the current standard of care, because of apparently higher detection rates and an earlier gestational age at diagnosis. First-trimester nuchal translu cency on ultrasonography forms the basis of this new form of screening, alt hough studies of its efficacy have yielded widely conflicting results, with detection rates ranging from 29% to 91%, Studies of first-trimester serum screening with measurements of pregnancy-associated plasma protein A and fr ee beta-human chorionic gonadotropin serum concentrations have been much mo re consistent, with Down syndrome detection rates of 55% to 63% at a 5% fal se-positive rate. The combination of first-trimester ultrasonographic and s erum screening has the potential to yield a Down syndrome detection rate of 80% at a 5% false-positive rate. although this approach has not been adequ ately studied. There have been no studies performed to date to directly com pare the performance of first-trimester and second-trimester methods of scr eening. Two major trials are underway that will address this issue, one in the United Kingdom and one in the United States. Until the results of these trials are available, the current standard of care with respect to Down sy ndrome screening should not be changed, and first-trimester screening shoul d remain investigational.